As others have said, remove the # to uncommit the line.
Commits are a special type of line in many languages that allow us humans to stick info (generally for humans) inside the code that the interpreter skips over. From the machines perspective this block looks like:
environment:
POSTGRES_PASSWORD: HDFnWzVZ5bGI
Note that the entire line is missing.
As a side note. Please change the password as it's been posted to the Internet.
Thanks, I tried with and without the POSTGRES_USER line commented out, still not joy. The documentation says it should default to default when not declared.
As for the password, don't worry I changed it right away.
I assume there is nothing in the database? Delete the file under volumes and relaunch. At a guess your database for initialized without a user and is now just in that state.